Salmonella enterica serovar Typhimurium (ST) is a leading cause of gastroenteritis in the United States and an emerging cause of invasive non-typhoidal salmonella in sub-Saharan Africa. This proposal will investigate the mechanisms by which IgA, the most abundant class of antibody in intestinal secretions, prevents ST from invading intestinal epithelial cells. In preliminary studies, we have demonstrated that a monoclonal IgA antibody (mAb) known as Sal4, which is specific for ST's O5-antigen, not only protects mice from mucosal ST infection, but it also attenuates ST virulence in vitro, without impacting cell viability or growth. Based on these and other results we hypothesize that cross-linking of the O-antigen by Sal4 triggers an outer membrane stress response that promotes the transition of ST from an invasive state to a non-invasive. However, before embarking on a more detailed mechanistic study of Sal4 and its influence on ST virulence, additional IgA mAbs against the O-antigen are required. Sal4 recognizes one of at least six different antigenic determinants on the ST's O-antigen. It is not known whether IgA antibodies against other O-antigenic determinants are protective or capable of suppressing ST virulence. Thus, Aim 1 is to generate a collection of IgA mAbs directed against distinct antigenic determinants on the ST O-antigen and test these mAbs for the ability to passively protect mice against intragastric ST infection. In Aim 2 we will characterize the IgA mAbs identified in Aim 1 for their abilities to attenuate ST virulence in vitro, using established assays that measure bacterial motility, invasion of intestinal epithelial cells, and exopolysaccharide production. These experiments will lay the groundwork for more detailed mechanistic studies aimed at understanding how secretory antibodies protect the intestinal mucosa against ST infection.